The present invention relates to a method for changing the natural frequency of a nip roll construction in a paper board machine. The present invention also relates to a nip roll construction in a paper or board machine, the nip roll construction including two rolls which form the nip.
In paper machines and in paper finishing devices, vibrations constitute quite a considerable problem, and in the present-day systems, in an attempt to achieve ever higher speeds, the problems of vibration have been manifested ever more clearly. There are several possible sources of vibration in paper machines, and some of the most important ones of these sources are rolls and cylinders, which comprise a large mass that revolves at a considerably high speed. Of course, in connection with manufacture, attempts are made to make the dimensional precision of rolls as good as possible and, moreover, the rolls are balanced in order to eliminate vibrations.
In present-day paper machines and paper finishing devices, however, ever increasing use is made, of rolls provided with soft coatings, which rolls may constitute a considerable source of vibration during operation. Such rolls are used, for example, in supercalenders, soft calenders, presses, coating devices, size presses, and equivalent, in which said roll provided with a soft coating forms a nip with another roll. Through the nip, a paper web and possibly a felt, wire or equivalent are passed. In such a nip roll solution, when a joint or splice of a wire, felt or web, considerable impurities, or something else that causes a considerable change in the thickness of the web passing through the nip runs through the nip during operation, the coating has to yield, in which case the coating acts as a spring that activates vibration. Owing to the visco-elastic properties of the coating, the coating is deformed so that it activates itself, i.e., in this case, no outside activating is needed. The deformation formed in the coating has not time to be reversed before the same point enters into the nip again, in which case the deformation is amplified and the roll starts resonating. The activated vibration itself is synchronized with a multiple of the frequency of rotation of the soft roll.
For example, in a size press and in a coating device of the type of a size press, the nip is formed by means of two rolls so that one nip roll has been mounted by means of bearing housings directly on the frame construction of said device, whereas the opposite roll has been mounted by means of its bearing housings on loading arms, which have been linked on the frame construction of the machine. In such a case, in particular, the roll mounted on loading arms starts vibrating, in which connection the coating on the soft-faced roll is deformed, as a result of which the vibration is intensified and the roll starts resonating.
One common prior-art mode of eliminating such vibrations is a change in the running speed of the machine so that, at the running speed concerned, the vibration is no longer intensified but starts being attenuated. Thus, problems of vibration have restricted the running speed of the machine.
In the applicant""s FI Patent 94,458, a method and an equipment are described, by whose means the critical speed of a roll can be changed in view of prevention of vibration. The critical speed of a roll can be changed by changing the mass of the roll and/or by changing the rigidity of the suspension of the roll and/or by changing the location of the roll suspension point in the axial direction of the roll and/or by changing the elastic constant of the journalling of the roll and/or by supporting the roll from the roll face by means of a displaceable support roller.
In the applicant""s FI Patent Application 971864, a method is described for attenuation of oscillation in a paper machine or in a paper finishing device by means of a dynamic attenuator, which comprises an additional weight suspended on the oscillating object by means of a spring. In the method, the oscillation frequencies of the oscillating object are measured constantly by means of one or several oscillation detectors. The measurement signals given by the oscillation detector are amplified by means of an amplifier and fed into an oscillation analyzer, which identifies the problematic excitation frequency and converts said problematic excitation frequency into a control signal. The control signal is fed into a control device, by whose means the elastic constant of the spring of the dynamic attenuator and/or the mass of the dynamic attenuator is/are changed in order to make the specific frequency of the dynamic attenuator substantially equal to the problematic excitation frequency. The dynamic attenuator may consist, for example, of a substantially horizontal bar attached to a bearing housing of the roll, an additional weight being suspended on said bar and the location of the weight on the bar being displaceable.
The method and the device in accordance with the invention are particularly well suited for use in a nip roll construction in which a roll nip is formed between a soft-faced roll and a hard-faced roll. In such constructions, rapidly increasing vibrations may arise through deformation of the coating on the soft-faced roll, and it must be possible to bring such vibrations under control.
This is accomplished in the present invention so that the elasticity of a hydrostatic bearing element of a nip roll and, thus, the resonance frequency are varied before the deformation has had time to proceed too far, in which connection the level of vibration can be kept under control. When a pressure accumulator is provided for the pressure medium of the hydrostatic bearing element, which pressure accumulator can be switched on and off, the elasticity of the bearing element can be varied.
The possibility of regulation of bearing elasticity in accordance with the present invention provides a simple mode of avoiding vibrations in rolls and, for example in calenders, of avoiding barring without having to change the process parameters.